U.S. Pat. No. 3,722,381 discloses a pair of eccentric shafts of the dual amplitude type arranged in the drum of a road roller. The eccentric masses of the eccentric shafts are identically configured. The eccentric shafts are arranged in cassettes, connected to the gable ends of the drum. A hydraulic motor drives/rotates one of the eccentric shafts which, in turn, transmit the rotation movement to the other eccentric shaft via an intermediate shaft. The eccentric shafts connection to the intermediate shaft is arranged in a way that gives the eccentric masses synchronized positions during the rotation. FIG. 1 of the patent shows a high amplitude position in which both the fixed and movable eccentric masses of the eccentric shafts are cooperating, that is, the position which the eccentric masses are intended to take in one of the rotation directions of the eccentric shafts. The rotating and cooperating eccentric masses are intended to actuate the drum of the roller to vibrate with highest possible amplitude in this position. FIG. 2 shows the low-amplitude-position which the movable eccentric masses take in the other rotation direction of the eccentric shafts. An imagined section of any of the eccentric shafts and its eccentric masses in the cooperating position shows a radial extension limited by a 5-sided geometrical shape. A drawback of the 5-sided shape is that it is not optimal from the viewpoint of mass-moment of inertia. It is also observed in the imagined section that a great deal of the section is taken up by balanced masses which does not contribute to the eccentric characteristics of the eccentric shaft but instead to an unwanted increasing of the mass-moment of inertia of the eccentric shaft. The same applies to the rings connecting and making the movable eccentric masses pivotal relative to the fixed eccentric masses. In both cases, the consequence is that the eccentric shaft is unnecessarily power and energy consuming during start up due to the shaft's high mass-moment of inertia.
FIG. 3 of the Chinese patent publication CN102995521 shows an eccentric shaft of dual-amplitude-type. The shaft is shown in a low-amplitude position in which its movable eccentric mass (at the bottom) partly balances the fixed eccentric mass (on top). The aforementioned problem, that parts of the eccentric shaft section do not contribute to the eccentric characteristics of the eccentric shaft, is solved by the fixed eccentric mass at the same time having a “carrying” function. A section through the eccentric masses would probably show that the sections are externally limited by circle shapes. The circle shapes provide a low mass-moment of inertia but the locations of the circle shapes, in relation to the point of intersection between the rotational axis and the section plane, are not optimal in this aspect. The circle shapes appear to be located at a distance from the rotation shaft that exceeds the diameter of the circle shape. In an imagined high-amplitude position, in which the movable and fixed eccentric masses are co-operating, it is also not optimal to have the eccentric masses distributed on two circle shapes in the section.